PROJECT SUMMARY Prior research has found associations between early childhood emotion regulation, executive functioning, and excessive or low-quality media use. However, these studies have been limited in that they only examine television and videos ? not increasingly prevalent mobile devices such as smartphones and tablets ? and that they assume a unidirectional relationship between media use and child development. Informed by the transactional nature of child development (i.e., Sameroff, 1975), this study aims to examine bidirectional, longitudinal associations between emotion regulation, executive functioning, and several aspects of child media use relevant to how children use modern mobile and interactive technologies. Particularly because mobile devices can be taken anywhere throughout a child's day and provide more short bursts of gamified play experiences, they may have a qualitatively different effect on child outcomes and need to be examined through updated methods such as mobile device-based passive sensing. This proposal's Specific Aims are to: Aim 1) Examine cross-lagged associations between emotion regulation (high emotional reactivity and intensity) and media use duration and use for calming purposes; Aim 2) Examine cross-lagged associations between executive function deficits (low inhibitory control, low emergent metacognition) and media use duration, rapid switching between apps, and lower-quality media content; and Aim 3) Examine mediators of the associations between media use and emotion regulation or executive function; specifically fewer parent-child activities and less non-digital play, respectively. The study will recruit 400 parents of children aged 3-4.99 to complete three waves of online surveys, including standardized measures of child emotion regulation (Child Behavior Checklist and the Rothbart Child Behavior Questionnaire ? Very Short Form), executive functioning (BRIEF ? Preschool Scale), and time diaries of media use and child activities at each data collection wave. In addition, child mobile device use will be measured through the innovative approach of mobile device-based passive sensing, which, through an application installed on family smartphones and tablets, will quantify the daily duration, frequency of app switching, and content of child app usage for 1 week at each data collection wave. Structural equation modeling will be used to examine concurrent and cross-lagged associations between child emotion regulation, executive functioning, and media use over the 6 months of data collection. This study will fill critical gaps in knowledge about early childhood media use and developmental and behavioral outcomes that can be translated into clinical practice guidelines, interventions for children with excessive media use, and improved digital design for children.